how to snap several vertices to the same z.position
You can select the vertices and then hit S+Z+0+Enter. Then they'll have the same Z-Position.

S is the shortcut key for "Scale". Z is the the axis to scale on and 0 is the amount to scale by.



Or, if you wanted to align the vertices to a specific vertex, you can:


Select the vertex you want to align to
Hit Shift+S and select Cursor to Selected.
Change the pivot point to 3D Cursor
Select the other vertices and hit S+Z+0+Enter.




Or alternatively (without the need to change the pivot point to 3D Cursor), you can


Select "Snap Element" on the 3D Header and select "Vertex"
Then select all the vertices except the one you want to align to
Hit G to grab
Then hit Z to constraint the translation to the Z axis
Hold Ctrl and hover the cursor over the vertex you want to align to until there's a circle over the vertex (don't hold Ctrl if the "Snap during transform" option is on)
Then click (or hit Enter)